1. Field of the Invention
This invention relates to an ignition apparatus for an internal combustion engine, and more particularly to an ignition apparatus making ignition using a beam of light.
2. Description of the Prior Art
An ignition apparatus generally used for an internal combustion engine is of the type in which a spark plug is disposed on the wall of the combustion chamber of the engine to generate spark discharge in response to the application of a high voltage thereby igniting the air-fuel mixture supplied into the combustion chamber.
The ignition apparatus of this kind has been defective in that accumulation of carbon on the surface of the insulator of the spark plug, wear of the spark plug electrodes due to repeated spark discharge, etc. result in a gradual decrease of the energy of spark discharge or in a difficulty of spark discharge until finally ignition of the air-fuel mixture becomes impossible.
In the selection of the spark discharge position of the spark plug in the ignition apparatus of the type described, the spark plug is generally disposed to discharge a spark at a most easily ignitable position taking into account the influence of various factors including the air-fuel ratio and the flow of the air-fuel mixture in the combustion chamber. Thus, for example, the spark plug is disposed to protrude to the center of the combustion chamber. Such a spark plug position has however been proved undesirable from the aspect of, for example, the durability of the spark plug.
To solve the above problem, an ignition apparatus has been proposed in which a light beam of high energy density such as a laser beam is used to ignite the air-fuel mixture. The proposed ignition apparatus has based on the method of directly radiating the energy of light to the air-fuel mixture. According to this method, however, it is difficult to ignite the air-fuel mixture, within a short time corresponding to the rotation period of the engine, by the temperature rise of the gas resulting from the absorption of the energy of light, since the light absorption factor of the gas is low. The ignition by the proposed ignition apparatus is generally resorted to the phenomenon of dielectric breakdown of the gas (by turning the gas into the state of plasma) in the field of energy of light of high energy density which is about several ten megawatts. For this purpose, a light source for generating a very high output, such as, a giant pulse laser is essentially required. The proposed ignition apparatus has therefore been defective in that the cost thereof is very high and it is not practical, since such a light source consumes a large amount of power and operates with low efficiency, and the scale of the apparatus becomes inevitably large.